Liking a post is a signal, especially when you're the first 'liker'. If it was posted by someone you don't see every day it's unlikely you'd immediately like so

Liking a post is a signal, especially when you're the first 'liker'. If it was posted by someone you don't see every day it's unlikely you'd immediately like something. People are like a heard of sheep, when others that starting something it makes them feel like they can too. These people that don't speak to you every day now feel like they can like it too and it's no longer a direct signal to you. Getting rid of the number of likes would change this dynamic (getting people to break down social barriers) but Facebook's answer is the 'Promote' button which really just changes the context of the post as it becomes an ad.